A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging using a projection system onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction
During operation of a lithographic apparatus, the elements of the projection system inevitably absorb a certain amount of energy from the beam and therefore heat up. This phenomena is known as “lens-heating” although it also applies to mirrors in reflective and catadioptric projection systems also. The term “lens-heating” will be used below but should not be taken as limiting the invention to use of refractive optical elements only or limited to heating of lenses only. As the elements of the projection system heat up, their shape and/or position may change, introducing aberrations and other errors into the projected image. This is of course undesirable and therefore a variety of efforts have been made to reduce or compensate for the effects of lens-heating. These efforts include cooling lenses, heating lenses with additional heaters to reduce thermal gradients, and introducing adjustable elements into the projection system to compensate for the lens heating effects.